f also doc the new splicing logic

Author: TheBlueMatt

lightning/src/sign/mod.rs

@@ -141,9 +141,9 @@ pub(crate) const P2WPKH_WITNESS_WEIGHT: u64 = 1 /* num stack items */ +
 pub(crate) const P2TR_KEY_PATH_WITNESS_WEIGHT: u64 = 1 /* witness items */
 	+ 1 /* schnorr sig len */ + 64 /* schnorr sig */;
 
-/// If a [`KeysManager`] is built with [`KeysManager::new`] with `v2_remote_key_derivation` set,
-/// the script which we receive funds to on-chain when our counterparty force-closes a channel is
-/// one of this many possible derivation paths.
+/// If a [`KeysManager`] is built with [`KeysManager::new`] with `v2_remote_key_derivation` set
+/// (and for all channels after they've been spliced), the script which we receive funds to on-chain
+/// when our counterparty force-closes a channel is one of this many possible derivation paths.
 ///
 /// Keeping this limited allows for scanning the chain to find lost funds if our state is destroyed,
 /// while this being more than a handful provides some privacy by not constantly reusing the same
@@ -1451,6 +1451,8 @@ impl ChannelSigner for InMemorySigner {
 	fn new_pubkeys(
 		&self, splice_parent_funding_txid: Option<Txid>, secp_ctx: &Secp256k1<secp256k1::All>,
 	) -> ChannelPublicKeys {
+		// Because splices always break downgrades, we go ahead and always use the new derivation
+		// here as its just much better.
 		let use_v2_derivation =
 			self.v2_remote_key_derivation || splice_parent_funding_txid.is_some();
 		let payment_key =